Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Fine tuning Q-jet
bob69:
Wow old cars, lots of good information to digest.
I agree each engine combination requires it's own tune. I have four different engines, each built for a different purpose. The tunes are different. Each is tuned to give the best idle, cruise and wot performance.
The first rule for carb tuning is to get the distributor timing correct. Before any carb adjustment I'll record the distributors timing from idle to 4000 rpm in 500 increments. I'm recording total advance (initial and mechanical) without the vacuum advance hooked up. I start with 10° initial and see what the total is. If total is greater than 36° the distributor is modified to get total to 36° or less. If timing advance starts before 1000 the advance weight springs are stiffened up. I only use vacuum cans with a 15° limit. All cars have the vacuum advance hooked up.
Would you use 16.0 afr target with e85? I think not. Calif has e10 but adds a boat load of oxygenators. As much as 10 - 15%. About four years ago calif mandated yet another fuel formula change. All my cars started running rough. More oxygenators less gas.
I had to retune all the carbs. For example, it took me two months and many tanks of "gas" to get the set up correct on one. I tested jets from 71 to 75. Settled on 74s as the performance was noticeably best. Even retested 73s and 75s to confirm 74s were the best. The point being that I use seat of the pants testing and not afr to tune a carb. Interestingly, the 74s gave the best overall afr's.
I agree that misfires will increase the o2 levels the afr testor see. I do not hear any pings but its possible. Will test with 91 to see if that helps. By the way, 91 goes for $7.00 a gallon around here. Consider yourself lucky you're not in calif.
Regarding your opening comments. I do not do things willy-nilly. The mab were changed following reciipe #1 on page 111. The idle circuit was not altered, based on comments found on this site. When I tried the large jets it was a last chance to test the set up before making any permanent changes elsewhere. Also, jets in the 71 - 73 range for 0.070 mab are recommended on recipe #1 on page 111. And finally, the oversized jets privided the best idle quality. Very smooth and afrs in the low 14s. With the mab and jet changes, the afr had to be 13.0 to get close to the same idle quality. Still not as smooth.
I still do not see any explanation why I see a momentary 1 second lean spike when the throttle is first opened. My other carbs show a momentary 1 second rich spike. Perhaps Cliffs comment that this carb may be a pos and use another carb is right. Just don't like writing it off without an explanation.
Kenth:
Carburetors are "stone-age" technology and need to be treated as such.
A/F meters or Lambda devices are too sensitive to evaluate what happens in a carb due to no computers for fuel/timing control.
What controls a carbs ability to deliver a proper combustable mixture due to atmospheric pressure into the void in intake are the various openings in each circuit inside the carb to mixture air and fuel adjusted to the current vacuum signal strenght. There will always be a delay, larger or smaller, depending on speed of throttle openings that the A/F meter/Lamba device will notice that in reality mean little or nothing for the performance of the stone-age carb.
There is seldom if ever needed to alter the main airbleeds.
Adjust if needed the main jetting from the main airbleed sizes.
Since you have Cliffs book take a look at pages 97-98 "Primary Main Fuel System" and you´ll get an idea.
Also, on page 99 Cliff describes the "tip in" procedure. Have you done this?
FWIW
old cars:
bob69
Answer this question. When you see the " lean spike" do you feel a hesitation / stumble
old cars:
--- Quote from: Kenth on March 18, 2022, 02:34:48 AM ---Carburetors are "stone-age" technology and need to be treated as such.
A/F meters or Lambda devices are too sensitive to evaluate what happens in a carb due to no computers for fuel/timing control.
What controls a carbs ability to deliver a proper combustable mixture due to atmospheric pressure into the void in intake are the various openings in each circuit inside the carb to mixture air and fuel adjusted to the current vacuum signal strenght. There will always be a delay, larger or smaller, depending on speed of throttle openings that the A/F meter/Lamba device will notice that in reality mean little or nothing for the performance of the stone-age carb.
There is seldom if ever needed to alter the main airbleeds.
Adjust if needed the main jetting from the main airbleed sizes.
Since you have Cliffs book take a look at pages 97-98 "Primary Main Fuel System" and you´ll get an idea.
Also, on page 99 Cliff describes the "tip in" procedure. Have you done this?
FWIW
--- End quote ---
Kenth
Not sure I am understanding your first paragraph. Can you elaborate
Cliff Ruggles:
I completely agree with Kenth about using A/F meters to tune these engines using carburetors and conventional distributors with VA.
Before you get your bricks out and start throwing them at me hear this out.
In any and ALL cases we tune for best results in every area. Idle tuning comes first, but it is done in conjunction with finding the ideal initial or base timing.
It's super easy to dial in the ideal idle mixture the engine wants, and you don't need a vacuum gauge or hand held tach. Just listen to the engine. Back the screws out till noticeably rich but not "blubbering". Turn them in until you just notice a speed change or the engine starts to become slightly "rough" or unhappy, then gently back out till it cleans up nicely and smooth again. Do both sides, "balance" them if/as needed, DONE with idle mixture. You can gander at your A/F meter at this point, but no matter what it says the reading is what your engine wants, likes, and responds best to.
I like to tune the primary main jet next. This may take a few tanks of fuel as described above but we need to NAIL DOWN the most ideal jet size for heavy throttle without the secondaries.
Move on to the primary metering rods. IF you have the idle fuel system nailed down and your "recipe" includes metering rods providing full control of A/F from rich to lean, which it should, start out lean (APT down/metering rods deep in the jets) and start driving the vehicle. Realize that the vacuum advance and mechanical advance are "players" here so running leaner mixtures will require the ideal spark lead to effectively burn them. I've spent days and even weeks nailing down the ideal settings for timing and part throttle APT settings.
I was always thinking that a lean mixture would net the best fuel economy, but found that with these things it's difficult, if not near impossible to get past the basic laws of physics. It's simply going to take a certain amount of energy to move a given mass a given distance. So going noticeably lean has NEVER once resulted in the best fuel economy combined with the best throttle response, acceleration, and power for all "normal" driving scenarios. I'll add here that I've installed A/F monitoring devices and was actually quite surprised by the readings I was seeing compared to the actual test results. This is where monitoring becomes difficult. Long-lengthy reads on the subject, following resident "experts" on Forums, and others who do testing in these areas throw monkey wrenches right into the middle of that deal.
What is my "target" A/F is the first thing you need to ask yourself? Am I looking for a "fixed" number, like 12.5 to 1 for WOT? Or will my very efficient and well thought out engine build be happier with 13.2 to 1? Or 12.8? How about part throttle A/F, is it a magic number like 13.5, or 13.7, or even 14.2 to 1?
I was seeing a WIDE range of A/F in the "normal" driving range at times going clear up over 17 to 1, and my engine still operated FLAWLESSLY. Throw that number into most of these conversations if you want get the natives all stirred up!
continued....
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